Abstract: The molecular concentration of a constituent such as oxygen of an anesthetic agent in a gas is measured by contacting the gas with the surface of a solid body, momentarily heating the surface so that the constituent reacts at the surface to alter a property of the surface such as reflectivity, and determining the degree or rate of alteration occasioned by the heating step. The momentary heating operation may be performed by light from a laser focused on to a small localized region. The body surface can include a thin film of a material such as a metal reactive with the gas constituent of interest at elevated temperatures. The preferred methods provide extraordinarily rapid response, and also form a permanent record of each measurement. Where the gas is at substantially constant pressure, the measured molecular concentration can be interpreted as an indication of the proportion of the constituent in the gas.